1. Field of the Invention
The present invention relates to silicon integrated circuit processing, and, more particularly, to a method of forming a semiconductor dielectric in the presence of both gettering and strengthening agents.
2. Description of the Related Art
In semiconductor processing, the formation of a gate dielectric typically includes using dry/wet/dry growth techniques to grow an oxide on a substrate surface. In order to form a high quality, low defect gate dielectric, many oxide growth techniques entail using a chlorine source in the process gas flow. In particular, chlorine and other halogens can be introduced to the substrate/oxide interface during oxidation to getter the metallic contaminants that are inadvertently deposited on the wafer surface. Gettering agents, such as chlorine, are used to passivate the metal contaminants and thereby reduce the metal""s tendency to attract loose electrons. Alternatively, gettering agents in some cases are used to vaporize the metal contaminants and lift the contaminants off the wafer surface altogether. Without the gettering process, metallic contaminants are known to attract electrons into the dielectric and adversely affect the threshold voltage of the gate structure. As such, gettering of metallic contaminants is essential for the formation of a high quality gate dielectric.
However, the gate dielectric thus formed can still be susceptible to hot electron degradation and dopant penetration which can reduce reliability of the gate dielectric and ultimately cause electrical failures in the device. A hot electron is a high energy charge carrier that is travelling in the channel region of the substrate between the source and drain that is attracted into the gate dielectric and can damage the gate dielectric thereby affecting the performance of the transistor. In particular, a gate dielectric formed of silicon dioxide, for instance, can trap hot electrons via the dangling silicon bonds. The trapped hot electrons have shown to degrade the oxide and erode its insulating properties. Similarly, the gate oxide can also permit dopants such as boron to diffuse into the underlying channel which can alter the threshold voltage of the channel and render the device less reliable.
To address the problems associated with gate dielectric degradation, nitrogen containing compounds are commonly incorporated into the gate oxide to strengthen the lattice structure of the dielectric so as to form a gate dielectric that is resistant to hot electron injection and boron diffusion. This gate dielectric strengthening process is commonly known as gate hardening and is typically performed after gate oxide formation. For instance, the gate hardening process for a gate dielectric that is primarily made of silicon dioxide (SiO2) typically comprises subjecting SiO2 to a gaseous oxy-nitride (NxOy) to form a silicon oxy-nitride SiOxNy gate dielectric. The nitrogen improves the lattice structure of the dielectric so that the dielectric is less susceptible to physical damages resulting from dopants or hot electrons. While the nitrogen incorporation process has shown to improve the gate dielectric properties, precise control of the nitrogen profile within the oxide layer is difficult to achieve because the nitrogen distribution largely depends on proper diffusion of the nitridizing species throughout the oxide.
To address this problem, a method has been developed to incorporate nitrogen in situ by bonding nitrogen to silicon and oxygen to form a SiOxNy dielectric from the outset as opposed to diffusing nitrogen through the dielectric after the formation of SiO2. The growth of an in-situ hardened gate oxide typically uses gaseous oxy-nitride NxOy as a nitrogen source during the oxide growth process whereas the conventional gate hardening process uses NxOy to diffuse nitrogen into an already formed oxide layer. As a result, the SiOxNy gate oxide formed using the in-situ method has a more consistent and controlled nitrogen profile. Moreover, the in-situ method incorporates nitrogen into the dielectric at the same time as oxide formation and therefore allows for a reduction in thermal budget and other related processing costs.
Disadvantageously, however, the presence of oxy-nitrides (NxOy) during gate oxide formation generally precludes the use of chlorine to getter metallic contaminants as chlorine can catalyze an explosive branching chain reaction between NxOy and Ox. As a result, gettering of metallic contaminants is usually not performed during the growth of in-situ hardened gate dielectrics because of the above mentioned safety concerns. As a consequence, the quality of the in-situ grown SiOxNy dielectric is generally inferior when compared to dielectrics that are formed using the conventional dielectric formation process where chlorine can be used during oxide growth to getter metallic contaminants.
Hence from the foregoing, it will be appreciated that there is a need for a method of forming an in-situ hardened dielectric in which chlorine can be safely used to getter metallic contaminants during the formation of the dielectric. Furthermore, it will be appreciated that there is a need for a method of using chlorine and nitrogen simultaneously to getter metallic contaminants and strengthen a dielectric lattice structure without creating a safety hazard. To this end, there is a particular need for a method of forming a nitrogen strengthened SiOxNy dielectric in which chlorine can be safely used as a gettering agent during the formation of SiOxNy.
The aforementioned needs are satisfied by the present invention which teaches a method of simultaneously strengthening and gettering a semiconductor dielectric. In one aspect, the present invention comprises a process for producing a gaseous gettering agent selected to getter contaminants in the dielectric and a gaseous strengthening agent selected to strengthen the dielectric. In particular, the gaseous strengthening agent is selected to inhibit rapid oxidation of the gaseous strengthening agent by the gettering agent. Furthermore, the process comprises exposing the dielectric to the gaseous gettering agent and the gaseous strengthening agent so as to strengthen the dielectric while simultaneously gettering the dielectric.
In another aspect, the present invention comprises a process for producing a nitrogen strengthened solid dielectric material in a semiconductor substrate. The process comprises producing a gaseous gettering agent and a gaseous nitridizing agent wherein the gaseous nitridizing agent is produced in a form that is not rapidly oxidizable by the gettering agent. Furthermore, the process comprises oxidizing the semiconductor substrate in the presence of the gaseous gettering agent and the gaseous nitridizing agent such that the gettering agent getters the contaminants and the nitridizing agent bonds nitrogen to the oxidized semiconductor substrate during the oxidation to form a dielectric comprising of the oxidized semiconductor substrate material and the bonded nitrogen. In one embodiment, the process can be applied to the formation of a gate dielectric while in another embodiment the process can be used to strengthen the dielectric of a capacitor.
In yet another aspect, the present invention comprises using an H2O steam based oxidation system containing dichloroethene (DCE) and ammonia (NH3) to form an in-situ hardened SiOxNy gate dielectric. Preferably, the oxidation system comprises 0.001%-1% by volume dichloroethene (DCE) combined with 0.002%-30% by volume ammonia (NH3) in H2O steam. Preferably, the percent volume of NH3 is at least twice the percent volume of DCE. In contrast to other commonly used in-situ gate oxide growth methods that use NxOy as a nitrogen source, the present invention in this aspect uses a steam based NH3 and DCE gas stream as a source for both nitrogen and chlorine during the oxide growth process. In particular, a portion of the NH3 undergoes a gas phase substitution reaction with DCE to release chlorine while the remaining NH3 is decomposed to release nitridizing species for in-situ formation of SiOxNy. Furthermore, the processing conditions are set to inhibit the formation of NxOy species that are potentially explosive in the presence of chlorine. As such, the present invention advantageously provides a source of chlorine that can be safely used during the formation of in-situ hardened SiOxNy gate dielectrics.
In one embodiment, the reaction can occur in a furnace batch system or rapid thermal processor (RTP). The gas stream comprising DCE, NH3, and H2O is introduced into the processing system via a hydrogen rich steam that is catalytically or pyrogenically generated. Furthermore, the processing temperature can be approximately between 650-1050xc2x0 C. and the processing pressure approximately between 30 mTorr-19000 Torr. However, it can appreciated that other equipment and processing parameters can be used without departing from the scope of the present invention. Advantageously, the nitrogen and chlorine sources provided by this embodiment are compatible and do not pose a safety hazard when mixed. Furthermore, the present process conserves thermal budget and processing costs by introducing nitrogen and chlorine to the processing environment via the same input stream.
In yet another aspect, the present invention provides a reactant gas that can be safely used to provide both chlorine and nitrogen simultaneously in a semiconductor processing environment. In one embodiment, the reactant gas comprises approximately 0.001% to 1% volume DCE, 0.002%-30% volume NH3, and H2O. Preferably, the percent volume of NH3 is greater than DCE as additional NH3 is needed to provide a source of nitrogen for the process. In one embodiment, the percent volume of NH3 is at least twice that of DCE to ensure that there is sufficient unreacted NH3 to source nitrogen to the system. Preferably, the DCE and NH3 undergo a gas phase substitution reaction in which chlorine is subsequently released. Furthermore, the remaining NH3 is preferably decomposed to release nitrogen to the processing environment. Furthermore, the temperature and pressure of the process are preferably set to inhibit the formation of NxOy. Preferably, the process temperature is approximately between 650-1050xc2x0 C. and the process pressure is approximately between 30 mT-19000 T. In one embodiment, the process is run at a temperature of 800xc2x0 C. and a pressure of 760T. Advantageously, the reactant gas can safely source both chlorine and nitrogen to a system and therefore permits the simultaneous introduction of chlorine to various nitrogen incorporation processes without posing a safety hazard.
In another embodiment, the reactant gas comprises 49% ammonia (NH3), 50% water (H2O), and 1% DCE (dichloroethene). Similarly, chlorine (Clxe2x88x92) is released when NH3 is induced to react with DCE and nitrogen (N2) is released from the decomposition of the remaining unreacted NH3. In particular, the nitrogen produced in this embodiment can be used to strengthen a capacitor dielectric comprising Si3Nx, wherein x less than 4, by transforming Si3Nx into Si3N4 while the chlorine is simultaneously used to getter metallic contaminants. Advantageously, the sources for nitrogen and chlorine provided by this embodiment are compatible from a safety standpoint and therefore can be used simultaneously to strengthen the lattice structure of the dielectric and getter metallic contaminants.
From the foregoing, it will be appreciated that the aspects of the present invention provide a safe method of simultaneously strengthening the lattice structure of a dielectric and gettering metallic contaminants on the substrate surface. In particular, the invention provides a novel method for forming an in-situ hardened SiOxNy gate oxide in which the nitrogen sources selected for hardening the dielectric will not be rapidly oxidized by the chlorine source used to getter contaminants. Thus, chlorine can be safely used to getter contaminants during in-situ hardened dielectric formation. Furthermore, the invention also provides a method of using nitrogen to strengthen an already formed Si3Nx dielectric while simultaneously using chlorine (Clxe2x88x92) to getter metallic contaminants. These and other advantages of the present invention will become more apparent from the following description taken in conjunction with the following drawings.